Hops have been used for centuries for the purpose of contributing a clean bitter taste to beer. Analysis of hop constituents has demonstrated that the most important bittering compounds derived naturally from hops are a group of analogous compounds generally known as alpha acids (humulones).
In a conventional brewing process, hop cones are boiled with sweet wort in a copper kettle for about one to two hours, and then the resulting wort mixture is filtered and allowed to cool. Isoalpha acids are produced by isomerization of alpha acids during boiling of the wort. The isomerization yield in the boiling process, however, is low. Such low yields have led to the development of commercial processes which extract the active compounds, such as, alpha acids and beta acids, from hops and convert them to the desired hop flavors, e.g., isoalpha acids, rhoisoalpha acids, tetrahydroisoalpha acids and hexahydroisoalpha acids, at relatively high yields. The commercially produced hop product may then be added post-fermentation to maximize utilization and provide consistent hop flavor.
It is well known in the brewing industry that isoalpha acids are susceptible to degradation upon exposure to light. This degradation in beer produces a highly undesirable skunk-like odor.
It is also well known in the art that hydrogenating the isoalpha acids (or the precursor alpha or beta acids) will increase the light stability of the beer. This hydrogenation requires a catalyst, such as palladium on carbon (Pd/C).
The hydrogenation of normal homolog isoalpha acids with platinum oxide is described by P. M. Brown, G. A. Howard and A. B. Tatchell, J. Chem. Soc. 545 (1959). However, the process results in low yields of tetrahydroisoalpha acid. E. Byrne and S. J. Shaw, J. Chem. Soc. (C), 2810 (1971) describes the reduction of tetrahydroisoalpha acid to deoxygenated products by hydrogenation with palladium on carbon in methanol at a pH of about 3. A process for isomerizing and hydrogenating alpha acids to produce tetrahydroisoalpha acid (THIAA) or hexahydroisoalpha acid (HHIAA) is described in U.S. Pat. No. 5,013,571, the disclosure of which is herein incorporated by reference. Further, U.S. Pat. No. 5,600,012 describes a single step process for making pure tetrahydroisoalpha acids from pure iso-alpha acids by a controlled hydrogenation process.
When alpha, beta, or isoalpha acids are hydrogenated, compounds which are referred to as "catalyst poisons," and which are associated with the hop extract, foul the catalyst. Such fouling of the catalyst causes the hydrogenation of the alpha, beta, or isoalpha acids to proceed slower than normal and eventually cease before all of the alpha, beta, or isoalpha acids are hydrogenated. In some instances, the catalyst becomes so quickly fouled that it must be replaced during the middle of the hydrogenation process in order for the hydrogenation to proceed to completion. Simple replacement or changeout of the catalyst is an unsuitable solution to the poisoning problem for at least two reasons. First, the substitution of a new catalyst midway through the hydrogenation process can promote the formation of overreacted products. Second, the catalyst is quite expensive, and frequent replacement of the catalyst greatly adds to the cost of production. Accordingly, there remains a need for a process for producing tetrahydroisoalpha acids hop flavors from alpha acids that is simple, inexpensive, and high yielding.